1. Field of the Invention
This invention relates in general to an improvement in an eyepiece for adjusting diopter and more specifically to an improvement in viewfinders for single lens reflex cameras for adjusting diopter.
2. Description of the Prior Art
In binoculars, microscopes and viewfinders of optical instruments and the like, the image formed at a given position by the optical system is generally observed with the eye. A large number of people have limited and variable focussing powers. For example, since a viewfinder for a single lens reflex camera is generally standardized to form images about -1 diopter (one meter ahead of the eye), it is often difficult for near-sighted, far-sighted or presbypopic persons to observe an image formed one meter in front of the eye through an unaided or naked eye. This leads to obvious difficulties in focussing a single lens reflex camera. While employment of spectacles avoids such problems with diopter, there is a likelihood that the visual field is eclipsed and is hard to be seen because the eye is held away from the eyepiece by the spectacles in front of the eye. Additionally, the contrast of the image scene is lessened by the introduction of ambient light. Several approaches have been suggested to provide diopter adjustment and also to adjust the position of an image in optical instruments in conformity to the individual difference in the adjusting faculties of the eye. One typical method is to move the eyepiece along the optical axis and this method has been actually practiced in the viewfinders of cine-cameras, binoculars, microscopes or the like. As a rule, if an eyepiece of focal length f is displaced by .DELTA.d, then the amount of varying diopter can be represented in terms of 1000.DELTA.d/f.sup.2 diopters. It is, accordingly, clear that a slight amount .DELTA.d of movement results in wide variations in diopter with eyepieces of a short focal length.
Additionally, in order to make an eyepiece movable to adjust diopter, there is a necessary space requirement for permitting movement in optical instruments. In view of the foregoing, the above mentioned means for adjusting diopter by movement of the eyepiece are effective only in optical instruments where the focal length of the eyepiece was relatively short or there was sufficient room for movement of the eyepiece.
Within a viewfinder for a single lens reflex camera, an image of an object formed on an objective lens and projected on a focussing glass plate is generally magnified for viewing by an eyepiece having a positive refracting power so that focal length of the eyepiece must be longer than the length of an optical path between the eyepiece and the focussing plate. However, a penta prism has to be interposed for reversal of an image between the focussing plate and the eyepiece in a conventional viewfinder of a single lens reflex camera. In the case of a 35 mm single lens reflex camera for example, the optical path length refined above is normally longer than 50 mm. Therefore, the focal length of the eyepiece is about 55-70 mm. Since a sufficient latitude of approximately 2 diopter is needed to overcome individual difference in diopter, analysis of the above diopter variations shows that the eyepiece should be moved within a range of about 6-10 mm in order to ensure a diopter latitude of 2 diopter when the focal length of the eyepiece is about 55-70 mm. However, in the case of a 35 mm single lens reflex camera, space for placement of the eyepiece behind the penta prism is too limited to allow movement of the eyepiece. Furthermore, a greater movement of the eyepiece results disadvantageously in an increasing rate of change in viewfinder magnification. These were the reasons why the prior art in the 35 mm single lens reflex cameras field failed to adjust diopter continuously. It is known that a successful prior art application was to selectively place adjustment lenses behind the eyepiece in accordance with the individual difference for the purpose of adjusting diopter. Nevertheless, this prior art approach needed several kinds of attachment lenses and had inherent difficulties in that the necessary visual field was partially eclipsed and hardly visible because of the positioning of the attachment lens behind the eyepiece.